A method has heretofore been known wherein the hydroformylation reaction of an olefin is carried out in the presence of a rhodium catalyst. Further, several methods have been disclosed for separating an unreacted olefin, aldehyde products and the catalyst from the reaction solution of hydroformylation reaction products.
For example, Japanese Unexamined Patent Publication No. 70634/1984 discloses a method for producing aldehydes which comprises reacting an olefin with carbon monoxide and hydrogen in the presence of a water-soluble rhodium-phosphine complex compound, and it discloses, as a method for recovering the unreacted olefin, a method of using a stripping tower by means of a synthesis gas (a mixture of carbon monoxide and hydrogen).
Further, INDICATIONS, Winter 1982/83 (The International Journal of Davy Mackee) discloses a method for producing butyraldehyde by reacting propylene with carbon monoxide and hydrogen in the presence of a rhodium catalyst having triphenylphosphine (TPP) as a ligand, and it discloses a process wherein by a gas stripping type reaction system, unreacted propylene, hydrogen, carbon monoxide and butyraldehyde are withdrawn as a gaseous effluent and condensed to obtain a liquid product of butyraldehyde containing propylene, which is directly charged into a stripping tower and contacted with carbon monoxide and hydrogen to separate and recover the unreacted olefin from the liquid product, and the recovered unreacted olefin is supplied together with the carbon monoxide and the hydrogen to the hydroformylation reactor. In this case, butyraldehyde discharged from the stripping tower is further subjected to a subsequent step of distillation for recovery of the dissolved gas.
Further, Japanese Patent Application No. 138630/1993 discloses a hydroformylation method of an olefin, which comprises reacting an olefin with carbon monoxide and hydrogen in a hydroformylation reaction zone in the presence of a rhodium catalyst having a trivalent organophosphorus compound as a ligand, wherein a liquid mixture derived from an effluent from the hydroformylation reaction zone, which contains an unreacted olefin, aldehyde products and the catalyst, is contacted with carbon monoxide and hydrogen to separate and recover the unreacted olefin from the reaction solution without substantially deactivating the rhodium catalyst, and the recovered unreacted olefin is supplied together with the carbon monoxide and the hydrogen to the hydroformylation reaction zone.
Thus, many proposals have been made for a method of countercurrently contacting a gas mixture of carbon monoxide and hydrogen to be supplied to the hydroformylation reaction with the product stream in a gas-liquid contact zone, when an unreacted olefin is to be recovered from the product stream in the hydroformylation process of an olefin.
An unrecovered dissolved olefin in the above gas-liquid contact zone will be lost or will have to be recovered by a further recovery means such as fractional distillation. Accordingly, the degassing efficiency in the gas-liquid contact zone has been substantially influential over the economy of the process.
In every one of the above methods, the amounts of carbon monoxide and hydrogen as the gas for recovery of the unreacted olefin must be controlled to meet the mass balance in the hydroformylation reactor, and the degree of freeness for a change of the amounts is limited. Accordingly, if the amount of a dissolved olefin contained in the product stream increases, the degassing efficiency deteriorates, and from the viewpoint of economy, there is naturally the upper limit for the olefin concentration in the hydroformylation reactor. On the other hand, from the viewpoint of the reaction efficiency, the olefin concentration in the hydroformylation reactor should better be high to attain a high reaction rate and high productivity.
Accordingly, it has been difficult by the conventional technology to increase the olefin concentration in the hydroformylation reactor without increasing the amount of an unrecovered olefin in the gas-liquid contact zone in the hydroformylation process of an olefin which comprises recovering an unreacted olefin from the product stream by countercurrently contacting the hydroformylation reaction products with a gas mixture of carbon monoxide and hydrogen to be supplied to the hydroformylation reaction in a gas-liquid contact tower.